Bipolar technology has been extensively used through the years for applications requiring high speed, high current drive, and low noise. Scaling of CMOS technology has not only drastically increased the density of CMOS integrated circuits, but it has also improved device and circuit performance. This rapid advance in MOS systems has placed increasing pressure on the ability of bipolar circuits to dominate high-performance applications. However, the same advanced fabrication techniques, such as advanced lithography and self-aligned schemes, apply to MOS devices has been also applied to bipolar devices themselves. Like scaling techniques implemented for MOS, bipolar scaling occurs in the vertical as well as the horizontal dimension. Low-energy ion implantation and preamorphization techniques have enabled thinner base regions to be constructed while tailoring the collector profile simultaneously. Rapid thermal annealing allows these thin layers to remain intact by limiting dopant diffusion through reduced thermal exposure.
Continued down scaling of homojunction bipolar transistors, however, has lead to practical limits dictated by the need to sustain sufficient current gain, among other requirements. As the base width is reduced, the base doping must be increased in order to maintain a reasonable base resistance and avoid punchthrough. Punchthrough is a condition in which the depletion regions at the base-emitter and base-collector junctions extend completely through the base, effectively shorting the emitter and collector. However, the base can be doped to only a certain level because of the significant loss in injection efficiency and thus current gain.
Moreover, as the base width in the bipolar transistor decreases, the collector-base junction voltage has an increasing effect on the neutral base width because the depletion region increases in width. This effect is sometimes referred to as base-width modulation and is more commonly known as the Early effect. The degree to which this effect impacts the operating characteristics of a bipolar transistor is represented by the Early voltage, V.sub.A (volts). A more thorough discussion of the Early effect can be found in STANLEY WOLF, SILICON PROCESSING FOR THE VLSI ERA, Vol. 2 (1990), which is incorporated herein by reference. It has been found that the value of V.sub.A can be increased by raising the doping level in the base. Higher doping decreases the penetration of the depletion region as a function of increasing collector-base reverse-bias voltage.
Typically, boron has been the primary p-type dopant in forming the base region. However, as device sizes have continued to further decrease, the possibility of using more efficient dopants has been explored. One such dopant is indium. It has been found that adding indium to a homojunction bipolar transistor results in increased collector currents and common-emitter transistor gains, excellent collector current saturation characteristics and an increase of the Early voltage when compared to boron-implanted transistors. The introduction of indium as the doping material for the base region significantly improves the function of bipolar transistors with little or no penalty in Early voltage and punch-through, see U.S. Pat. No. 5,681,763 to Ham, et al, which is incorporated herein by reference. Additionally, it is believed that the use of indium minimizes the spreading of the reverse-biased collector-base junction depletion region into the transistor base, i.e., base-width modulation.
During the indium implantation into the silicon substrate, the indium achieves a natural concentration distribution within the silicon substrate. Unfortunately, however, the impact of the indium atoms on the silicon crystalline structure damages the structure such that the damage must be annealed out. The high temperature anneal, unfortunately, has an adverse effect on the natural indium concentration distribution. Thus, it is highly desirable to avoid the high temperatures associated with present fabrication processes.
Accordingly, what is needed in the art is an improved method of fabricating a bipolar transistor which eliminates the problems associated with homojunction bipolar transistors.